The spot fleets are groups of spot instances of different types, managed much like AutoScaling groups, but with a different API and configuration mechanism. Each instance type needs to be explicitly configured with a certain bid price and weight, so that the group's capacity can be scaled out over various instance types.
These groups are quite resilient because they are usually spread over multiple spot instance types, so it's quite unlikely that the price will surge on all of them at once. But much like the default AutoScaling spot mechanism they are also unable to fall back to on-demand capacity in case the prices surge across all their instance types. MaxSpotter will also try to avoid using all instances of the same type, in many cases, with enough capacity by spreading over three or four different spot market price graphs, which in addition to the on-demand fallback capability should be also quite resilient in the event of spot terminations.